The features, introduced in February 2017, are designed to prevent browser exploits from executing malicious code.

“An application can directly load malicious native code into memory by either 1) loading a malicious DLL/EXE from disk or 2) dynamically generating/modifying code in memory,” Microsoft explained. “CIG prevents the first method by enabling DLL code signing requirements for Microsoft Edge. This ensures that only properly signed DLLs are allowed to load by a process. ACG then complements this by ensuring that signed code pages are immutable and that new unsigned code pages cannot be created.”

Fratric showed that the ACG feature can be bypassed and informed Microsoft of his findings on or around November 17. The company had initially planned on patching the vulnerability with its February Patch Tuesday updates, but later determined that “the fix is more complex than initially anticipated.”

Microsoft now expects to release a fix on March 13, but the date exceeds Google Project Zero’s 90-day disclosure deadline so the details of the vulnerability have been made public. Project Zero has classified the flaw as having “medium” severity.

This is not the first time Project Zero has disclosed an unpatched vulnerability found by Fratric in Microsoft’s web browsers. In February 2017, it made public details and proof-of-concept (PoC) code for a high severity type confusion issue that could have been exploited to crash Internet Explorer and Edge, and possibly even execute arbitrary code.

The security hole, tracked as CVE-2017-0037, was fixed by Microsoft in March 2017, roughly two weeks after it was disclosed.

Fratric is the creator of a fuzzer named Domato, which last year helped him uncover tens of vulnerabilities in popular web browser engines.

Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.